Metal treatment – Process of modifying or maintaining internal physical... – Heating or cooling of solid metal
Reexamination Certificate
1999-06-14
2002-02-26
Ip, Sikyin (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
Heating or cooling of solid metal
C148S697000
Reexamination Certificate
active
06350329
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method for producing fine-grained alloys, particularly fine-grained 6xxx aluminum alloys which exhibit superplasticity, and to the alloys produced by the method.
BACKGROUND OF THE INVENTION
The advantages of superplastic properties in metals are well known, and particularly well employed in the automotive and aerospace industry. Because of their fine-grained microstructures, superplastic metals and alloys may exhibit from several hundred percent to several thousand percent elongation without necking when pulled in tension at temperatures 20 exceeding 0.5 T
m
, where T
m
is the absolute melting temperature of the material. In contrast, non-superplastic metals and alloys typically elongate less than 100% before necking under similar conditions. Accordingly, superplastic metals may be formed into a multitude of complex shapes not achievable with other metals.
Currently, commercial interest in the aerospace and automotive industries is focused on superplastic forming (“SPF”). SPF is a manufacturing process which exploits the phenomenon of superplasticity by using low gas pressures (less than about 1000 psi (7 MPa)), and concomitantly low energies, to form parts having complex shapes. This process reduces part counts and the need for fasteners and connectors, reducing product weight and manufacturing costs. In addition, SPF may be performed using a single surface tool in a single forming operation, thus reducing tooling costs. The advent of SPF therefore increases the potential commercial applications in which superplastic materials may be employed.
Superplastic behavior in metallic alloys may be described by the equation
&sgr;=k{acute over (&egr;)}
m
where &sgr;=flow stress, k=material constant, {acute over (&egr;)}=strain rate, and m=strain rate sensitivity. In superplastic metals, m usually ranges from about 0.4 to 0.8. “Quasi-superplastic” metals and alloys have m values of around 0.33. Materials having m values less than 0.3 are considered to be non-superplastic.
Most metals and alloys capable of achieving superplasticity must be specially processed for superplasticity. The microstructures of such metals and alloys may be refined through thermomechanical processing to impart such properties to the material. For a material to be superplastic, it is typically refined to possess an equiaxed, fine-grained structure, typically with grains about 20 &mgr;m or less in diameter and preferably about 10 &mgr;m or less. In addition, for such a material to be commercially useful, it must be statically stable such that its grains do not experience significant growth at superplastic forming temperatures. Where the thermomechanical process for refinement includes static recrystallization, which is a common component of such processes, a weak or random texture and the presence of predominantly high-angle grain boundaries is also required. The development of thermomechanical processes effective for creating alloys having such properties has proven to be extremely challenging.
An extensive amount of research has been conducted in an effort to discover thermomechanical processes useful for producing superplastic alloys, including aluminum alloys. This work has resulted in the development of several superplastic alloys, but undoubtedly, many commercially important superplastic alloys have yet to be discovered. In particular, although several superplastic 2xxx, 5xxx, 7xxx and 8xxx aluminum alloys have been produced, there has been a significant deficiency in successful research concerning the grain refinement and superplasticity of 6xxx aluminum alloys. New superplastic 6xxx aluminum alloys would be particularly desirable, because 6xxx alloys are highly weldable, corrosion resistant, extrudable and low in cost compared with other aluminum alloys. Thus, there is a need for the development of methods for imparting superplastic properties to alloys, particularly 6xxx aluminum alloys.
Of the 6xxx aluminum alloys, 6061, 6063, 6066, and especially 6013 and 6111, possess substantial promise for extensive use in the aerospace and automotive industries. Indeed, non-superplastic aluminum alloy 6013, a medium strength, age-hardenable alloy developed by ALCOA in the early 1980s, has been selected for use on Boeing Co.'s state-of-the-art 777 aircraft, as well as for many other automotive and aerospace applications. This is not surprising, given the favorable properties of this alloy and the fact that it can be processed to develop properties superior to other 6xxx alloys. For example, it has corrosion resistance superior to that of 2xxx and 7xxx aluminum alloys, which are heavily used for aerospace applications. The yield strength of 6013-T6 is 12% higher than that of 2024-T3, it is nearly immune to corrosion that results in exfoliation and stress-corrosion cracking, and it is 25% stronger than 6061-T6. In addition, the alloy 6013-T4 has better stretch-forming characteristics than other aerospace aluminum alloys. Accordingly, there is a need for the development of methods for imparting superplastic properties to 6061, 6063, 6066 alloys, and particularly to 6013 and 6111 aluminum alloys.
To date, efforts expended to impart superplasticity to 6xxx aluminum alloys have not been very successful. U.S. Pat. No. 4,092,181 to Paton, et al., which describes what is known in the art as the “Rockwell process,” discloses a method for imparting a fine grain structure to aluminum alloys having precipitating constituents. The thermomechanical process of the Paton, et al. method consists of solution heat treating such an alloy, overaging the alloy, then subjecting the alloy to a particle-stimulated nucleation (“PSN”) process during which the alloy is mechanically worked and recrystallization is induced. Although the Paton, et al. patent provides several examples of the method described therein, it does not describe the microstructures produced by the method, nor does it suggest that superplastic results were achieved. Indeed, experimental evidence available in the literature indicates that the method disclosed by Paton, et al. is not very useful for imparting superplasticity to 6xxx alloys. This is confirmed by the work performed in connection with the present invention, as described below.
Similarly, Washfold, et al. attempted to grain refine a 6063 aluminum alloy through PSN in order to induce superplasticity. See Washfold, et al., “Thermomechanical Processing of an Al—Mg—Si Alloy,” Metals Forum (1985) at 56-59. The thermomechanical process used is very different than that employed in the present invention, and consists of a solution heat-treatment followed by slow cooling to an overaging temperature, overaging, slow cooling to room temperature, cold or warm rolling, and static recrystallization with a slow heat-up to the recrystallization temperature. Washfold, et al. produced a microstructure exhibiting a minimum grain diameter of 10.5 &mgr;m (in the rolling plane), as measured using optical microscopy (“OM”) techniques. They obtained a maximum elongation of 148% at 450° C., due to significant grain growth occurring at 500° C. and above, within the superplastic forming temperature range. The Washfold, et al. process did not achieve superplasticity.
Kovacs-Csetenyi, et al. attempted to use compositional variation and thermomechanical processing to refine the grain structure and improve the superplastic performance of aluminum 6066 and three variants of aluminum 6061. See Kovacs-Csetenyi, et al., “Superplasticity of AlMgSi Alloys,” Journal of Materials Science 27 (1992) at 6141-45. The thermomechanical process used consists of solution heat-treatment followed by overaging, rolling, and static recrystallization, and bears no resemblance to that of the present invention. Kovacs-Csetenyi, et al. report strain rate sensitivity values in the range of 0.4 for each of the four alloys processed, as studied using temperatures between 500° C. and 570° C. and strain rates of 10
−3
to 10
−6
s
−1
, indicating that some
Crooks Roy
Starke, Jr. Edgar A.
Troeger Lillianne P.
Foley & Lardner
Ip Sikyin
LandOfFree
Method of producing superplastic alloys and superplastic... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method of producing superplastic alloys and superplastic..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method of producing superplastic alloys and superplastic... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2979762